In such electronic ballasts, voltages of several 100 V having frequencies of approx. 30 kHz to 1 MHz are switched by means of the constant-current transformers. In this situation, constant-current transformers are converters wherein current is taken from or alternatively fed back into the supply voltage during each operation cycle. Two important representatives are the buck converter and the boost converter. Such converters include a converter throttle, a converter diode and a converter switch, whereby the converter throttle is coupled serially between the first input connection and the load circuit. With regard to the high-frequency switching of high voltages described above, both conducted and also emitted electromagnetic interference are produced, which must lie beneath the limit values of the relevant EMC regulations. A distinction is made between common-mode interference, so-called Y-interference, on the one hand, wherein the charge equalization is effected in phase by way of the mains power lines, and differential-mode interference, so-called X-interference, on the other hand, wherein the charge equalization is effected out of phase by way of the mains power lines.
In order to reduce conducted common-mode interference, current-compensated chokes are often used in the power input. In difficult cases this measure is however often not sufficient. A method for the active compensation of common-mode interference is known from EP 0 763 276 B1 which provides a further improvement. Based on the teaching of this publication, it is for example possible to generate a voltage opposite in phase to the high-frequency half-bridge voltage. Since both voltages couple capacitively into the environment, the interference caused thereby is compensated for on account of their opposite phasing. In particular in constant-current transformers, as are used in electronic ballasts according to the invention, this principle has however not been successfully applied hitherto because the generation of a voltage opposite in phase is possible only with a high resource requirement. In particular in the case of frequency components above 1 MHz, the compensation no longer functions satisfactorily because the currents involved are not arbitrarily phase-locked in consequence of the capacitances involved.
Particularly problematical is the already mentioned conducted common-mode interference in equipment belonging to protection class 2, which must observe the limit values of the relevant EMC regulations without a metallic housing and without a protective conductor connection. In contrast to this, in equipment having a metal housing an internal charge equalization up to a certain level is made possible by way of the metal housing.